Recently, communication services with large communication capacity, including video service for smartphones, have been provided. Accordingly, there is a need for greatly increasing the existing communication capacity, and as a method of greatly increasing communication capacity, there are a method of increasing the bit rate used for optical communication and a WDM (wavelength division multiplexing) method that is a method of simultaneously transmitting optical signals having various wavelengths through one optical fiber. In the WDM method, a WDM method using two wavelengths of an optical signal at a band of 1310 nm and an optical signal at a band of 1550 nm were widely used, but recently, WDM (DWDM; Dense WDM) having a very dense frequency separation of 100 GHz and 50 GHz is adopted. Further, in order to further increase optical communication capacity, a method of increasing the bit rate of an optical signal having one wavelength and a WDM method of sending light having various wavelengths through one optical fiber have started being simultaneously applied.
However, in optical communication that modulates the intensity of laser light by supplying a current corresponding to “1” signal and “0” signal and analyzes signals according to a change in intensity of the light as “1” and “0” signals in a semiconductor laser diode, chirp in which laser light generated by a semiconductor laser diode chip changes in accordance with the magnitude of an injection current is generated. The “1” signal generally means a signal of a bit with large light intensity and a signal of light with small light intensity is called a “0” signal. In a semiconductor laser diode chip, more optical output is generated when the amount of an injected current is large, so the “1” signal described above corresponds to the case when a relatively large current flows in a laser diode chip, and the “0” signal corresponds to optical output the case when a relatively small current flows in a laser diode chip. For example, at a modulation speed in the 10 Gbps class, a wavelength change of about 5 GHz to 10 GHz is generated between a “1” signal and a “0” signal and such a wavelength difference is called chirp. In common DFB-LD, a “1” signal increases in frequency by about 5 GHz to 10 GHz in comparison to a “0” signal, so the wavelength of the “1” signal is shorter than the wavelength of the “0” signal. In an optical fiber, the propagation speed of light depends on the wavelength of the light due to dispersion and the dispersion changes the transmission speed of a “1” signal and a “0” signal in accordance with a chirp characteristic that is generated when a semiconductor laser is driven by “1” and “0”, so when an optical signal reaches an optical receiver, the “1” signal and the “0” signal are mixed and it is difficult to separate the signals.
This phenomenon is more serious, particularly when a bit rate is high and the transmission distance is large, and accordingly, optical transmission of an optical signal generated in a semiconductor laser at a band of 1550 nm driven in the 10 Gbps class to 10 Km or more is very difficult and even optical transmission to 5 Km is difficult in some cases.
In order to operate a semiconductor laser diode chip at a high speed in the 10 Gbps class, it is required to supply a bias current corresponding to a “0” signal and a modulation current corresponding to a “1” signal, in which a bias current flows to the semiconductor laser diode chip in response to the “0” signal and a current produced by adding the modulation current to the bias current flows in response to the “1” signal.
For high-speed communication in the 10 Gbps class, optical response of a semiconductor laser diode to a signal having an RF (radio frequency) frequency of 10 Gbps need to be quick, but it is preferable to increase a bias current in order to increase optical response to an RF electric signal of a semiconductor laser diode. The magnitude of the modulation current depends on the characteristics of an electronic circuit driving a semiconductor laser diode chip and it is preferable to decrease the magnitude of the modulation current in order to give an electronic circuit a high frequency response characteristic. Accordingly, in order to improve an RF response characteristic of a semiconductor laser diode, a bias current flowing to the semiconductor diode chip is increased, and when it has a modulation current magnitude of a low current to improve an RF characteristic of a driving circuit of a semiconductor laser diode, the difference in intensity of an optical signal corresponding to “1” and an optical signal corresponding to “0” becomes small. The ratio of the intensity of an optical signal corresponding to “1” and the intensity of an optical signal corresponding to “0” is called an ER (extinction ratio). When the ER is low, “1” and “0” signal are mixed at an optical receiving terminal due to chirp in a semiconductor laser diode chip and dispersion in an optical fiber, so it becomes difficult to decode an optical signal at the optical receiving terminal. It is possible to reduce mixing of optical signals due to chirp in a semiconductor laser diode chip and dispersion in an optical fiber by increasing the ER, but it is required to decrease the bias current and increase the modulation current in order to increase the ER. However, when the bias current decreases, the optical response speed of the semiconductor laser diode chip to an electric signal decreases, and when the magnitude of modulation increases, the response speed of a driving circuit for driving the semiconductor laser diode chip decreases.
In order to solve this problem, Chang-Hee Lee et al, have shown that longer-distance transmission is possible, as compared with when laser light outputted from a semiconductor laser diode chip is not optically filtered, by improving ER in a way of removing or reducing a “0” signal by optically filtering laser light outputted from a DFB-LD (Distributed feedback laser diode) light source in CLEO' 95 (CLE) 1995, CTul10). When a transmission band wavelength of an optical filter is set to “1”, a “0” signal having a longer wavelength than the “1” signal is blocked by the optical filter, so the intensity of the “0” signal transmitted to an optical fiber is weakened relatively to the “1” signal, so the ER increases and an optical receive easily receives an optical signal, and accordingly, it is possible to transmit an optical signal to a longer distance. Therefore, the line breadth of the transmission band of an optical filter needs to have a difference in transmittance at a meaningful level in accordance with the difference in wavelength of a “1” signal and a “0” signal and the difference in transmittance can be adjusted to the transmission band line breadth of the optical filter. As described above, since there is a frequency difference of about 5 GHz to 10 GHz between a “1” signal and a “0” signal, the line breadth of the transmission wavelength band of an optical filter is required to be set to show a meaningful difference in transmittance for such a level of wavelength difference.
In the reference, CLEO' 95(CLEO 1995, CTul10) by Chang-Hee Lee et al., 13 dB bandwidth of the transmittance band of an optical filter is set to 12 GHz, but it is preferable to use an appropriate value at 5 GHz to 30 GHz for the transmission bandwidth of an optical filter. For the optical filter, an optical filter having one transmission wavelength band peak at a wavelength band of at least 10 nm to 100 nm can be used, but a filter having a plurality of transmission wavelength bands at the wavelength band may be used. In an optical filter having a plurality of transmission wavelength bands, the −3 dB bandwidth described above is defined as a −3 dB bandwidth of any one transmission wavelength peak. When it has a plurality of transmission wavelength bands, the frequency differences between the plurality of transmission wavelength bands should be larger at least than −3 dB width of the transmission wavelength band.
On the other hand, in a DFB-LD type of semiconductor laser, the wavelength depends on the operation temperature, and it usually has a wavelength change rate of about 0.1 nm/° C. Accordingly, a semiconductor laser diode chip has a wavelength change of about 12.5 nm in accordance with an environment temperature change of −40° C. to 85° C. Therefore, when an adjacent wavelength drops by about 20 nm, it is possible to remove mixing of wavelength even if the temperature of a semiconductor laser diode chip is not adjusted. Accordingly, an optical signal having a wavelength separation of 20 nm or more is generally used without the temperature of a semiconductor diode chip adjusted. However, when the wavelength separation from an adjacent wavelength is within 10 nm, it is required to keep the temperature of a semiconductor laser diode chip constant, using a thermoelectric element in order to suppress a temperature change of the semiconductor laser diode chip.
In high-speed optical communication in the 10 Gbps class, chirp in a DFB-LD and dispersion in an optical fiber are generated regardless of the operation temperature of a semiconductor laser diode chip, so for long-distance high-speed optical communication in the 10 Gbps class, it is required to optically filter optical signals outputted from a semiconductor laser diode regardless of the wavelength separation between wavelengths used in the optical communication.
Further, the product of SFP (small form factor pluggable), which is an optical communication module that is being internationally standardized at present, has a very small internal standard, so there is a need for a compact optical element. At present, there are package housings such as a TO (transistor outline) type, a mini flat type, and a butterfly type as packages for mounting a semiconductor laser chip, in which the TO type package is very small in volume and its price is relatively very low, so it is actively used for an optical communication network for a subscriber that requires a large quantity. However, a package in which an optical filter for increasing an ER that is the ratio of “1” signals and “0” signals by optically filtering laser light emitted from a DFB-LD chip and a DFB-LD chip is disposed in an existing TO type package has not been reported yet.
Since the oscillation wavelength changes in accordance with the operation temperature in a semiconductor laser diode chip, in order to effectively transmit a “1” signal and effectively block a “0” signal, in the laser light signals emitted from a semiconductor laser diode chip, it is required to keep a predetermined relationship between the wavelength of laser light emitted from the semiconductor laser diode chip and the transmission band wavelength of an optical filter, in accordance with a change in environmental external temperature. If it is not, the “1” signal that is supposed to be transmitted is blocked and the “0” signal that is supposed to be blocked is transmitted well, so optical communication becomes difficult.
As a prior art document, there is Korean Patent Publication No. 10-1124171 (Feb. 29, 2012).